1. Field of the Invention
The invention is concerned with conversion of relatively heavy hydrocarbon streams to produce lower molecular weight materials from a portion or all of the charge. In a very broad sense of the term, such processes involve a "cracking" reaction in the sense that hydrocarbon or substituted hydrocarbon molecules are converted to reaction product of lower molecular weight.
2. Description of the Prior Art
The cracking may be of a general nature in that molecules of varied nature are converted, i.e., branched and straight chain aliphatics, naphthenes, aromatics, etc. The compounds so converted may also include other atoms in the molecule: metals, oxygen, sulfur and/or nitrogen. In particular processes, the intent may be to convert a certain class of compounds in order to modify a characteristic of the whole. Exemplary of the latter type of conversion is shape selective conversion of straight and slightly branched aliphatic compounds of 12 or more carbon atoms to reduce pour point, pumpability and/or viscosity of heavy fractions which contain these waxy constituents. The long carbon chain compounds tend to crystallize on cooling of the oil to an extent such that the oil will not flow, hence may not be pumped or transported by pipelines. The temperature at which such mixture will not flow is designated the "pour point", as determined by standarized test procedures.
The pour point problem can be overcome by techniques known in the art for removal of waxes or conversion of those compounds to other hydrocarbons which do not crystallize at ambient temperatures. An important method for so converting waxy hydrocarbons is shape selective cracking or hydrocracking utilizing principles described in U.S. Pat. No. 3,140,322 dated July 7, 1964. Zeolitic catalysts for selective conversions of wax described in the literature include such species as mordenite, with or without added metal to function as a hydrogenation catalyst.
Particularly effective catalysts for catalytic dewaxing include zeolite ZSM-5 and related porous crystalline aluminosilicates as described in U.S. Reissue Patent 28,398 (Chen et al.) dated Apr. 22, 1975. As described in that patent, drastic reductions in pour point are achieved by catalytic shape selective conversion of the wax content of heavy stocks with hydrogen in the presence of a dual-functional catalyst of a metal plus the hydrogen form of ZSM-5. The conversion of waxes is by scission of carbon to carbon bonds (cracking) and production of products of lower boiling point than the waxes. However, only minor conversion occurs in dewaxing. For example, Chen et al. describe hydrodewaxing of a full range shale oil having a pour point of +80.degree. F. to yield a pumpable product of pour point at -15.degree. F. The shift of materials from the fraction heavier than light fuel oil to lighter components was in the neighborhood of 9% conversion.
Among the less specialized techniques for producing products of lower molecular weight than the hydrocarbon charge stock are catalytic cracking and catalytic hydrocracking. Catalytic cracking involves contacting the heavy hydrocarbon charge with a porous acidic solid catalyst at elevated temperatures in the range of 850.degree. to 1000.degree. F. to yield the desired lower boiling liquid product of greater value than the liquid charge (e.g. motor gasoline) together with normally gaseous hydrocarbons and coke as by-products. Hydrocracking employs a porous acidic catalyst similar to that used in the catalytic cracking but associated with a hydrogenation component such as metals of Groups VI and VIII of the Periodic Table. An excess of hydrogen is supplied to the hydrocracking reactor under superatmospheric pressure at lower temperature than those characteristic of catalytic cracking, say about 650.degree. F.
Since the introduction of zeolite catalysts as exemplified by U.S. Pat. No. 3,140,249, a large proportion of the capacity for catalytic cracking and hydrocracking has been converted to use of such highly active catalysts. The high activity zeolite catalysts are characterized by very low content of alkali metal. Sodium, for example, is present as a cation in synthetic faujasites by reason of their manufacture. Expensive ion exchange operations are carried out in the preparation of cracking and hydrocracking catalysts from synthetic faujasite to replace the sodium or other alkali metal by protons or poly-valent metal cations.
It has been recognized that such zeolites can function as catalysts when containing a moderate percentage of sodium. Thus Kimberlin and Gladrow Reissue patent 26,188 exhibits data showing cracking activity of a faujasite from which only one-third of the sodium has been removed by ion exchange. The extremely high activity of such catalysts as zeolite ZSM-5 has been moderated for specialized purposes by using the zeolite in the partially sodium form. See, for example, U.S. Pat. No. 3,899,544.
Zeolite ZSM-5 preparation is described in U.S. Pat. No. 3,702,886 which also describes several processes in which the zeolite is an effective catalyst, including cracking and hydrocracking. That zeolite is shown to be prepared from a forming solution which contains organic cations, namely alkyl substituted ammonium cations. Those large organic cations then occupy cationic sites of the zeolite and block pores at least partially. The conventional method for removing the organic cations is to burn them out with air at elevated temperature, leaving a proton at the site previously occupied by the organic cation. Sodium, or other alkali metal, at other cationic sites may then be ion exchanged to provide protons or multivalent metals as desired to prepare catalysts for cracking, hydrocracking and other purposes.
Regardless of the type of catalyst used, the principal products desired in cracking, hydrocracking and like processes for reducing molecular weight of hydrocarbon fractions are relatively low-boiling liquids such as motor gasoline, diesel fuel, jet fuel, No. 2 fuel oil and the like. Gaseous products such as hydrogen, methane, ethane, propane, etc. represent degradation of a portion of the charge to less valuable fuels than the desired premium products. In addition to being less valuable fuels, these gases require high proportions of hydrogen which can only deprive premium liquid products of hydrogen needed for their constitution.